This invention relates to an organic powder composition and a method of using the powder composition for forming a crosslinked paint coupled to a metal substrate for protecting the substrate from corrosion. More particularly, the powder composition includes a homogeneous mixture of a thermosetting resin and a solid, non-hydrolyzed, functional organosilane coupling agent.
It is known to form a composite coating on steel sheet for providing corrosion protection. U.S. Pat. No. 5,108,793; incorporated herein by reference, discloses rinsing a steel sheet with an alkaline solution containing a silicate, drying the sheet to form an insoluble inorganic silicate coating, rinsing the coated sheet in an aqueous solution containing an organosilane coupling agent and then drying the sheet forming a silane film on the outer surface of the silicate coating. The silicate coating provides excellent corrosion protection of the steel sheet. If the sheet is painted, the silane film acts as a coupling agent forming a strong covalent bond between the silicate coating and the paint. An embodiment of this patent includes cold-rolled and galvanized steel sheets initially treated with a phosphate conversion coating, then treated with the two step silicate/silane rinse coating and lastly electrostatically coated with a powder paint or cathodically electrocoated with a paint for forming a composite coating. The silicate/silane layer improves corrosion protection and strengthens the bond between the inner phosphate layer and the outer paint layer. A disadvantage with this process is that some of the silicate inevitably becomes transferred to the silane rinse tank thereby contaminating the silane solution. This contamination may reduce the effectiveness of the silane as a coupling agent. Another disadvantage is that hydrolyzed silane solutions have a relatively short storage life. Organosilanol solutions tend to condense and polymerize thereby minimizing their chemical interaction with paints.
It is well known to provide corrosion protection to steel sheet by coating with a liquid resin containing an organosilane coupling agent. These liquid resins have the same disadvantage referred to above in that hydrolyzed silanes have reduced effectiveness as coupling agents.
More recently, it has become known to provide corrosion protection to steel sheet by coating the sheet with a powder containing a silicone resin. U.S. Pat. No. 5,077,354 discloses an in-situ formed polymeric mixture comprising an acrylic graft copolymer of silicone resin used as a polymeric binder in powder paint. Silicone resin and ethylenic monomers are dissolved in a solvent heated to 50.degree.-150.degree. C. and mixed until polymerization is completed. The solvent then is stripped away leaving a non-gelled polymerized binder. A disadvantage of using this powder paint is believed to be that a good bond is not formed between the paint and steel substrate. Since the silicone resin has already been reacted when in-situ combined with the ethylenic monomers when forming the acrylic polymer, the silicone resin will not act as a coupling agent between the outer acrylic layer and the steel substrate.
Accordingly, there is a need to provide an organosilane having a long storage life that can be used as a coupling agent when forming a crosslinked coating. There is a further need for developing a low cost, environmentally safe process for forming an adherent crosslinked coating coupled to a metal substrate such as cold-rolled steel or galvanized steel.